In work we have just setup 3 monitors on a single machine. Why the big woop? Well untill i tried this, i was copnvinced that Eyefinity was the best way to go for this, but even Eyefinity is not without its issues. It seems that unless you have a DVI, HDML and DisplayPort monitor, then you could run into issues!! http://www.tomshardware.co.uk/forum/279498-15-displayport-adapter-eyefinity
However if you were just after simple multiple displays and you dont have multiple PCIe slots (think of most work PC’s here, not home PC’s) then the answer is multiple PCI cards
We used 2 8400GS cards, installed inthe first and second PCI slots. The Windows 7 driver didnt detect all the cards and the second card would not start. So i installed the latest nvidia drivers, and boom, they were both on, but my third monitor would not be detected. I have to hit the “Detect my displays” for the third and fourth options to come available
And that was it, job done

